U.S. patent application Ser. No. 405,258, filed Sep. 11, 1989 in the name of Rimai et al, and U.S. Pat. No. 5,023,038, issued Jun. 11, 1991 in the name of Aslam et al disclose a method and apparatus for fixing a multicolor toner image carried on a heat softenable outside layer of a receiving sheet. The receiving sheet is passed across a preheating plate to raise the temperature of the thermoplastic layer to or above its softening point. It is fed into a pressure nip created by a pressure roller and a belt or web backed by a heated roller. The belt or web is of a hard ferrotyping material such as stainless steel, nickel or the like. Relatively high pressure is applied between the belt and pressure roller to embed much or all of the toner image in the thermoplastic layer fixing the image. Some of the toner may be not entirely embedded but may be fused on the top of the layer, but with much of it embedded, the hard ferrotyping belt provides a photographic quality with an absence of relief and a high gloss. The image and heat softenable layer are retained in contact with the belt as it moves away from the pressure nip. The belt and receiving sheet are allowed to cool until the heat softenable layer is below its glass transition temperature. At this point it can be separated without offset. All this is accomplished without the use of offset-preventing substances like powders or liquids which would reduce the photographic quality of the image.
Other belt fixing devices are generally known for fixing toner images to plain paper. See, for example, European Patent Application 0301585, published Feb. 1, 1989; European Patent Application 0295901; and U.S. Pat. No. 3,948,215.
In some of the embodiments shown in the Rimai and Aslam applications an endless belt is heated above the glass transition temperature of the heat softenable layer of the receiver and then cooled below that temperature on each revolution. In "Belt Fusing Device", Research Disclosure, July 1990, page 559, it is suggested that a heat pump be used to transfer heat from the portion of the belt being cooled to a portion of the belt approaching the heated roller. Heat pumps are complex and prohibitively expensive for such applications. See also, U.S. Pat. No. 3,356,831 to Andrus et al, issued Dec. 5, 1967, which shows a heat exchange unit between an incoming portion of a web entering an electrophotographic fusing apparatus and the outgoing portion of the same web to reuse some of the fusing energy to preheat the receiver.